1. Field of the Invention
The present invention relates to an inner-rotor-type brushless motor in which a sensor board for detecting rotation of a rotor is attached to a bus bar which connects together U-phase, V-phase, and W-phase windings wound on a stator magnetic pole core and connects them to output wires.
2. Description of the Related Art
In an inner-rotor-type brushless motor, a sensor attached to a stator detects magnet poles provided on the rotor side so as to switch and control currents supplied to stator windings. When such a brushless motor is used for driving electrical tools or other electrical equipment which requires supply of relatively large current to the motor, a bus bar has conventionally been used for connecting wiring from an external power source to the windings.
The brushless motor is configured such that the bus bar attached to the stator and the sensor for rotation detection are individually arranged in the thrust direction. FIG. 12 shows a longitudinal cross section of a motor disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-125353. The illustrated motor includes a cylindrical rotor rotatably assembled into a housing via bearings at opposite ends of the housing; a stator coaxially disposed around the rotor; and a sensor for detecting the rotational phase of the rotor. The stator includes a cylindrical core; a plurality of windings wound on the core; a bus bar (a coil connection structure) which connects respective end portions of the windings; and electricity supply terminals which are connected to the bus bar.
The disclosed motor has a problem in that the bus bar and the sensor component individually arranged in the thrust direction increases the entire length of the motor, to thereby increase the size of the motor. Further, since the sensor must be attached to the housing, the number of components increases, thereby increasing cost and the number of manufacturing steps. In addition, the disclosed motor has a structure in which the number of cumulative errors increases, and thus, the sensor encounters difficulty in attaining high sensing accuracy. Such a structure decreases torque and increases torque ripples.
There is also known a motor having a structure in which a sensor is mounted onto a bus bar so as to reduce the size of the motor (see Japanese Patent Application Laid-Open (kokai) No. 2006-94573). FIG. 13 shows a portion of the structure of a stator which is fitted into a housing of a brushless motor disclosed in this publication. Conductive wires are wound on the core of the stator so as to form windings (not shown). The bus bar has a cylindrical portion which is provided on the lower side thereof and fitted into a central opening of the core. Lead wires extending from unillustrated windings corresponding to teeth are connected to respective connection terminals. The bus bar is positioned in relation to the core by means of a plurality of leg portions provided on the outer circumference of the bus bar.
Meanwhile, Hall elements are inserted into and held in corresponding recesses of a sensor holder; the leads of the Hall elements are inserted into holes of a circuit board which carries other electronic components; and the sensor holder is then fixed to the circuit board. Thus, the sensor elements mounted on the bus bar can detect magnetic poles by detecting sensor-dedicated magnets provided on the rotor side.
However, the conventional motor has a problem in that copper bars and connection terminals embedded in the bus bar generate heat, and lower the reliability of the sensors attached to the bus bar. The copper bars embedded in the bus bar serve as internal wiring for connecting leads from the windings to the corresponding connection terminals. A current (several A to several tens of A) for driving the motor flows into the windings via the copper bars and the connection terminals so that a motor torque is output. At that time, the copper bars and the connection terminals generate heat. Therefore, in the case where a semiconductor sensor, which is weak against heat, is mounted on or in the vicinity of the copper bars and the connection terminals, the reliability (in terms of breakage and service life) of the sensor lowers, and deteriorated sensor waveform causes a drop in torque, as well as an increase in torque ripples.